Thank you for the updates.
From your login screen, looks like your Authentication Profile has Identity First selected.
The login-id prompt only has the email address field, but the login prompt has the email address and password field.
This article explained the prompts.
That could be the reason that you don’t see the description changed after updating the Login page because your screen is the Login-id page.
Do you want to give it a try by updating the login-id page with the below scripts?
PUT request:
https://{domain}/api/v2/prompts/login-id/custom-text/en
{
"login-id": {
"title": "xxxx",
"description": "xxxx",
"buttonText": "xxxx"
}
}